In connection with automatic or automated variable-speed transmissions or clutches, it is known to derive a measurement magnitude by way of sensors, which indicates the position of an actuator. But since a sensor for detecting the position of an actuator may be defective, the actuator position indicated by the sensor is not unconditionally reliable. Thus, it is very advisable, in connection with the safety monitoring of automatic or automated change-speed transmissions, to check the actuator position.
From DE 101 37 597 A1, a method for fault diagnosis in a clutch actuator is known. In this method, a control value is applied to an input of the clutch actuator. As a function of this control value, a corresponding electric signal for the measured actuator position is produced at an output of the clutch actuator. From the same control value, an actuator model unit calculates an expected actuator position and sends it to the output of the actuator model unit. The signals for the measured actuator position and the expected actuator position are compared with one another in a comparing unit. The signal, present at the output of the comparing unit, is sent to a computer unit, which produces a corresponding status signal at its output. If there is approximate agreement, between the signals for the measured and estimated actuator positions, the computer unit concludes on the basis of an evaluation algorithm that the clutch actuator is functional and the sensor for determining the position of the clutch actuator is also functional. In contrast, if there is a large difference between the signals for the measured and expected actuator positions, it is concluded that a fault exists in the clutch actuator and/or the sensor for determining the position of the clutch actuator.
In the fault diagnosis method described, the measured actuator position is compared with a model-based actuator position. Since the model for calculating the actuator position deviates from reality, for example because the behaviour of the system cannot be modelled exactly, at each calculation step there is a discrepancy between the actual and the calculated and therefore expected position of the clutch actuator. To avoid a progressive drifting apart of the actual and calculated actuator positions from one another, in DE 101 37 597 A1 the output signal from the comparing unit, which is formed from the measured and calculated actuator positions, is fed back to the input of the actuator model unit and evaluated.
If, now, an error of the measured actuator position occurs, this is transposed into the calculated actuator position by the feedback of the comparing unit's output signal into the actuator model, and so to the calculated actuator position. The disadvantage of this is that because of the feedback to the actuator model the erroneous actuator position is retained in the model value, so errors of the measured actuator position are sometimes not recognized.
A clutch model is usually based on the fact that the position change is calculated continuously by virtue of a value actuation and this calculated value is integrated. Since the clutch model is calculated by a digital computer, position changes are only detected at discrete time points. These position changes relate to the time interval between two calculation steps. Consequently, instead of integrating the position change it is necessary to carry out a summation. To be able to solve this equation with a digital computer, the differential equation must be converted to a difference equation. Since the clutch model usually deviates from reality, for example because the behavior of the system cannot be modeled exactly, at each calculation step there is a deviation between the actual and the calculated position of the clutch actuator. This makes it evident that there is in principle a difference between the calculated and the measured position, which adds up with the passage of time.
The purpose of the present invention is to indicate a method for fault detection in an actuator, which ensures reliable monitoring of the position of the actuator and which eliminates the disadvantages of the prior art.
The objective addressed by the invention is achieved by a method of the type in question for fault detection in an actuator.